1. Field of the Invention
The present invention relates to a lens driving device for moving an optical lens in the direction of an optical axis and an imaging device using this lens driving device.
2. Description of Related Art
Typically, imaging devices such as video cameras equipped with an autofocus function and electric zoom function are provided with lens driving devices for moving a moving lens for focusing and a moving lens for zooming along the direction of an optical axis. Electromagnetically driven actuators having, for example, a coil and a magnet, such as electromagnetically driven devices, are commonly used as this type of lens driving device.
Conventionally, drive means employing a stepping motor or DC motor where rotational movement of the motor is converted to rectilinear movement using gears etc. so as to cause a zoom lens or focus lens to move along an optical axis direction are widely adopted as actuators used in these lens driving devices. However, rectilinear driving means using linear actuators that combine flat plate magnets and drive coils have also been adopted to accompany advancement of performance required in high-speed drive and high-precision drive.
As disclosed in patent document 1 and patent document 2 mentioned below, such an actuator is equipped with a magnet being flat and having an equal width in the drive direction, a yoke being flat and having an equal width in the drive direction, and a drive coil. A gap between the surface of the driving magnet and an opposing drive coil is always kept a fixed distance over the whole of a drive stroke region as a result of a moveable part being guided by a guide shaft.
As shown in FIG. 15, at the magnet that is flat and has the same width in the drive direction, flux density flowing from the surface thereof is such that the direction of the flux density tends towards the drive direction side at the upper surfaces at both ends of the magnet and thrust generated at the coil is therefore also at an angle with respect to the drive direction. The thrust in the drive direction generated by the actuator therefore becomes lower towards both ends in the drive direction and becomes higher at a central section, as shown in FIG. 16.
The moving lens is constrained in the drive direction by a guide shaft and there is a certain angle between the drive direction and the thrust direction. Load due to friction between the guide shaft and the moving lens therefore increases due to a thrust component being generated in a direction rectilinear with the drive direction, and the drive load at both ends of the magnet increases.
Patent Document 1: Japanese Patent Laid-open Publication No. Heisei. 7-239437.
Patent Document 2: Japanese Patent Laid-open Publication No. 2002-169073.
When the moving lens is driven, the thrust in the drive direction generated by the drive coil changes according to the position of the moving lens. This damages the linearity of the thrust, and the drive load also changes according to position, and this is detrimental to the drive performance for the moving lens.
In particular, in the case of a servo system lens driving device configured using feedback control using a position detector etc., the stability of the servo system fluctuates according to the position of the moving lens because of the problem described above, with this being detrimental to the servo characteristics.
Further, it is not possible to effectively utilize the entire length of the magnet because the thrust generated by the coil at the ends of the stroke direction of the magnet falls. In cases where, for example, the thrust generated at the ends is not sufficient for that required by design for an actuator, it is necessary for only portions where the required thrust is obtained to be utilized, and the entire length of the magnet is therefore not used effectively. This makes it necessary to make the drive stroke small, and prevents the device from becoming smaller.